The reliance on fossil fuels by power utilities and industrial applications (e.g., iron, steel and cement manufacturers) results in the unwanted production of many pollutants. For example, the emitted exhaust gas generated from thermal power plants using coal, LNG and petroleum or exhaust gas from gas turbine generators, incinerators, etc. are enormous. The exhaust gas may contain many pollutants including, for example, carbon dioxide, mercury, sulphur compounds, nitrogen compounds and carbon compounds. These pollutants are known to contribute significantly to atmospheric pollution. For example, carbon dioxide emissions are harmful for several reasons including their contribution to global warming, a trend that may lead to rising sea waters, droughts and agriculture disasters. Nature, Geological Society of London, Mar. 15, 2001.
The rise in greenhouse gas emissions, particularly carbon dioxide, has been attributed to the increased use of fossil fuels. It has been estimated that 22 gigatons (Gt) of carbon dioxide are emitted per year as a result of the use of fossil fuels. Worldwide, carbon dioxide emissions from coal-fired power plants account for about 1.8 Gt C/yr, of the total 6 Gt C/yr. Coal is the fuel most widely used for the generation of electricity worldwide because it is readily available, easily transportable, and relatively inexpensive. Indeed, approximately 70% of all the electricity used in the United States is generated from coal and natural gas. The increasing effects of greenhouse gases, such as carbon dioxide, have challenged the industrialized world to find new and better ways to meet the increasing need for energy while reducing the emission of pollutants.
Mercury is another pollutant that is particularly harmful to human health. In fact, ingestion of large amounts of mercury results in almost immediate death due to heart attack. Each year approximately seventy-five tons of mercury embedded in coal are delivered to fossil fuel burning power plants in the Province of Ontario. The combustion of the coal results in approximately two thirds of the mercury (i.e., 50 tons) emitted into the air over the Province of Ontario each year. A typical dispersion pattern of mercury over the Province of Ontario is illustrated in FIG. 1. As a result of the prevailing westerly winds, flue gases containing mercury may be disbursed from Ontario over great distances including much of New York State and New England.
An area that has been of particular concern is the release of sulphur compounds into the atmosphere during the refining of petroleum, the sweetening of natural gas, the processing of ore and the destructive distillation of coal. In particular, sulphur dioxide discharge occurs primarily due to the combustion of sulphur-containing fossil fuels, petroleum refining, manufacturing of sulphuric acid and smelting of sulphur containing ores. The major origin of sulphur dioxide is from sulphur bearing coals burned in the process of producing electrical energy. Sulphur dioxide has been reported to be responsible for damage to agricultural crops and the natural lakes throughout the world, notably in the U.S. and Canada.
In addition, the burning of fossil fuels also creates emissions of nitrogen and oxygen compounds that may be harmful. When these by-products enter the atmosphere, they may be transformed into acid and returned to earth as acid rain precipitation. Nitrogen emissions are particularly problematic because they form particulate matter in the atmosphere such as combinations of dust, soot and other solid-liquid compounds that restrict visibility and contribute to haze.
Another natural result from burning fossil fuels, particularly coal, is the emission of fly ash. Fly ash is generally considered to be mineral matter suspended in combustion gases. For each unit of pulverized coal burned, 60 to 80% of the particulate mineral matter is discharged in the flue gas as fly ash. Although fly ash may be relatively harmless, fly ash emissions have received particular attention because they can be observed leaving smokestacks and are generally considered to be unsightly.
Typically gas purification systems combine several basic processes into one overall system to remove pollutants. For example, refuse burning processes for treating, e.g., municipal waste, provides for the removal of particulates from flue gas by means of electrostatic filters or fabric filters and the partial removal of gaseous pollutants from the gas by acid or alkali treatment. Traditional fossil fuel burning systems also consist of relatively simple air pollution control equipment. Typically the system directs the hot combustion flue gases sequentially through a selective catalytic reducer, an electrostatic precipitator, a wet scrubber and, finally, into a smokestack for distribution into the atmosphere.
The selective catalytic reducer is used primarily to reduce concentrations of nitrogen compounds (e.g., NOx) and oxygen compounds by passing flue gases through a catalytic converter. The gas reacts with the ammonia of the catalyst to convert the nitrogen compounds and oxygen compounds into nitrogen gas and water. Selective catalytic reactors are practical for the removal of nitrogen compounds and oxygen compounds, but not for the removal of carbon dioxide, sulphur dioxide, mercury or fly ash.
The electrostatic precipitator is used primarily to reduce fly ash in the flue gas. The flue gas is passed through pipes containing negatively charged plates that impart a negative charge on the fly ash particles. The particles are then routed past positively charged plates, or grounded plates, which attract the now negatively charged fly ash particles. The particles adhere to the positive plates and can be collected for disposal. A typical electrostatic precipitator is illustrated in FIG. 2. Electrostatic precipitators are large and expensive to maintain, and are only effective for the removal of the particulate matter that comprises fly ash and not for the removal of gaseous pollutants, such carbon dioxide, sulphur dioxide or mercury.
Wet scrubbers are used to control sulphur dioxide omissions through a process known as wet flue gas desulphurization (FGD). Wet FDG systems remove gaseous sulphur dioxide from flue gas by absorption. The sulphur dioxide is contacted with caustic slurry consisting typically of water and lime. The lime reacts with the sulphur dioxide to form gypsum (CaO+SO2→CaSO4). Some gaseous compounds of mercury may also be absorbed in the liquid slurry; however, the absorption efficiency of mercury will depend on the speciation of the mercury. Thus, wet scrubbers are useful primarily for the removal of sulphur dioxide and not for the removal of other gaseous pollutants, such carbon dioxide, or mercury.
Sequestration has been proposed as one possible method for the removal of pollutants. For example, carbon dioxide sequestering comprises forming carbonates from readily available minerals. The concept of mineral carbonation is based on the idea that forming magnesium or calcium carbonates from most minerals is exothermic and thermodynamically favored. The resulting carbonates are stable solids that are known to be environmentally benign.
Lackner et al. have reported the sequestering of carbon dioxide through the formation of carbonates using magnesium or calcium bearing silicate minerals, e.g., serpentine. One implementation is based on an HCl extraction in which magnesium is obtained as MgCl2. The HCl is quantitatively recovered and the MgCl2 is transformed into Mg(OH)2. The hydroxide is carbonated in a gas-solid reaction that provides the energy consumed in recovering the HCl, resulting in an overall exothermic process. Lackner et al. also reported the possibility of direct carbonation of serpentine and provides an analysis of the thermodynamics of the process. See Lackner et al., Mineral Carbonates as Carbon Dioxide Sinks, which can be found as http://www.netldoe.gov/publications/proceedings/98/98ps/pspa-8.pdf.
However, there is a need in the art to provide a cost effective process to reduce pollutants, such as, carbon dioxide, mercury, sulphur compounds, nitrogen compounds and carbon compounds, which are produced during the combustion of fossil fuels and during other industrial applications. Moreover, there is a need in the art to provide a process that will allow the use of coal as a fuel source since it is readily available and relatively inexpensive in comparison to alternative energy sources.